Evergreen and deciduous tree species show distinct strategies to synchronize with seasonality in mid-elevational forests of central Himalaya

Abstract

Synchronisation of the leaf phenological patterns with favourable environmental conditions and the variations in the leaf traits allow tree species to coexist, support biodiversity and maintain forest productivity with a larger implication on the global carbon cycle. However, within a forest whether plants with contrasting phenological attributes follow similar or distinct adaptive strategies for resource gain in response to seasonality and how they avoid niche-overlap to coexist is less clear. Further, whether the trade-offs between faster leaf growth within a shorter duration and vice-versa as proposed in the worldwide leaf economic spectrum (WLES) holds at the local scale remain inconclusive. Here, we aim to (1) describe patterns of temporal variation in leaf exposure (i.e. initiation, the peak of exposure, longevity, and expansion period) and leaf structural properties (i.e. area, dry mass, specific leaf area, dry-mass lost, construction cost, and mass acquired before the rainy season) among 12 coexisting evergreen and deciduous trees; and (2) explore whether the predictions of the WLES framework hold for evergreen and deciduous mid-elevation trees of the central Himalaya range. Results suggest that with an earlier leaf flush/leaf initiation and faster leaf recruitment in deciduous tree species, these plant species may maximize resource gains while investing less in structural parts of the leaves. In contrast, evergreens exhibit a higher investment in structural traits and retain leaves for a longer period and thereby maximizing resource gains from the growing season. The principal component analysis showed that PC-1 and PC-2 cumulatively explained 68.4% of the total variation in leaf traits. There was a distinct grouping of evergreen and deciduous species along the principal axis-1 for trait variation, suggesting a niche separation, which may allow species to coexist. When tested for the significance of differences in the average values of leaf traits, we found that deciduous trees had higher average values for mass before the rainy season (MRS), dry mass-loss (DML), specific leaf area (SLA), and leaf area (LA) than evergreens. The leaf life span of evergreens was significantly higher compared to the deciduous form which may allow evergreens to pay back the leaf construction cost which on average is higher than deciduous tree species. In a way, deciduous may adopt the resource acquisition strategy while evergreen exhibits a resource conservation strategy. The proposed generalized conservative-acquisitive traits axis of the world leaf economic spectrum thereby holds true for mid-elevational tree species in the central Himalaya.